In recent years, increasing requests for bandwidth and quality of service on the part of users have prompted researchers to develop novel architectures and novel protocols for optical communications networks.
The WDM (wavelength division multiplexing) optical systems used in classic optical communications networks are not able to process current bandwidth requirements in an optimal manner, especially because of the great increase in the number of optical-electronic-optical conversions and multiplexing/demultiplexing operations and also because of the lack of flexibility in the occupancy of this bandwidth (high granularity, poor bandwidth occupancy).
In order to meet bandwidth needs, “all-optical” communications network architectures have been developed and appropriate optical switching techniques have been designed in order to make maximum use of the bandwidth offered by these architectures.
In the prior art, various classic techniques of optical switching are known: optical circuit switching circuits, optical packet switching and optical burst switching.
The architecture of optical networks implementing the technique of optical burst switching have been introduced by M. C. M. Qiao and M. J. S. Yoo, in: “Optical Burst Switching (OBS): “A new paradigm for Optical Internet”—Journal of High Speed Networks (JHSN), Vol. 8, to compensate for the lack of reliability of optical circuit switching networks and the immaturity of optical packet switching techniques. Indeed, the technique of optical circuit switching is relatively simple to implement but suffers from a lack of flexibility in the face of fluctuating data traffic and the change in state of communications channels. The technique of optical packet switching is, for its part, conceptually ideal but highly complex to implement because it requires especially the presence of numerous delay lines and fast processing of the headers of the optical packets.
The OBS technique consists in assembling a plurality of optical data packets arriving at an input node (or peripheral node) of the network and intended for a same destination node of the network in a single group of data packets, called an optical data burst, and in routing this burst up to the destination node. Relay nodes (or intermediate nodes) enable the burst to be relayed from the input node to the destination node.
Unlike in the optical packet switching technique, the OBS technique enables the transmission of more data by means of bursts travelling through the optical network (a burst by itself comprising a plurality of optical data packets). Thus the technical constraints at each node of the network are reduced in view of the smaller number of headers to be processed. Furthermore, the OBS technique is able to make efficient use of the capacity (in terms of occupancy rates) of the communications channels.
Today, there are mainly two known embodiments of the OBS technique.
A first known embodiment, known as separate control channel OBS or BCP (burst control packet) OBS relies on the preliminary dispatch of a control packet on a dedicated transportation channel separate from the transportation channel of the optical data stream with which it is associated. The pieces of information contained in the control packet enable the node of an optical network to reserve the optical resources that are necessary for it to route the burst towards another node of the network.
In general, the term “collision” (or contention) of bursts is used when at least two optical data bursts take the same wavelength and seek to access the same output port at the same time.
A second known embodiment called “in-band” control channel OBS or label OBS consists of the transmission, on a same transportation channel, of a label situated at the head of the optical data stream, this label containing the control data.
The above-mentioned two known embodiments (separate control channel and in-band control channel) implement comparable mechanisms to resolve burst collision in the time, spectral and space domains.
The resolution of collisions in the spectral domain consists in modifying the wavelength of one or more bursts in collision so that all the bursts take the same output port simultaneously. This especially requires that each node of an optical network should be provided with wavelength converters.
Now, the use of such wavelength converters requires the mobilizing of considerable electrical resources. Besides, it limits the degree of transparency of the nodes that implement them, since the conversion mechanisms implemented are not transparent to the data modulation formats using especially phase modulation or polarization modulation. In other words, there is no conversion technique at present (apart from optical-electronic-optical conversion) that enables functioning with phase-modulated and polarization-modulated data. Consequently, the nodes comprising such wavelength converters are not totally transparent for the data.
Furthermore, although an optical data burst undergoes no electronic conversion (the burst is simply conveyed by nodes in the optical network), the control packet (or the label) associated with it nevertheless mobilizes energy resources because of the electronic processing applied to it (optical-electronic-optical conversion, extraction and analysis of control data, decision making, etc).